An aerogel is a gel which has a lower density than the fully condensed form of the material comprising the gel. Aerogels typically are produced by replacing the liquid of a gel by air or another gas without allowing complete collapse of the structure. The seminal report on this was made by Kistler in 1931 (Nature, 127, 741 (1931)), who described the goal of the research as being “to test the hypothesis that the liquid in a jelly can be replaced by a gas with little or no shrinkage.” This early work led to aerogels through the use of supercritical fluids to extract liquid, and it led to the hypothesis that the gel structure itself can be preserved in the supercritical drying process, as disclosed by Marshall in U.S. Pat. No. 285,449 (1942).
There have been many successes in the aerogel field, as disclosed in the scientific and technical literature and in patents. Of relevance to the current disclosure is the area known in some contexts as Organically Modified Ceramics, referred to as ORMOCERS or called CERAMERS, which have been widely studied. A descriptive review of this area is that of R. C. Mehrotra (Present Status and Future Potential of the Sol-Gel Process, Chapter 1 in Chemistry, Spectroscopy and Applications of Sol-Gel Glasses, Structure and Bonding Series 77, Eds. R Reisfeld and C. K. Jorgensen, Springer-Verlag, Berlin, 1992). This reference points to the distinction between composite materials that are mixed at the molecular level and those that have mechanically combined components. This reference also discusses work directed to organically modified gels in the form of aerogels and their subsequently dried, fused, oxidized and otherwise treated forms. Also of relevance are works concerning aerogels and their applications. The book Aerogels edited by J. Fricke (Springer Proceeding in Physics 6, Springer-Verlag, Berlin, 1985), the book Sol-Gel Science, The Physics and Chemistry of Sol-Gel Processing by C. J. Brinker and G. W. Scherer (Academic Press, Inc. Harcourt Brace Jovanovich, Publ., New York, 1990) and the book Sol-Gel Technology for Thin Films, Fibers, Preforms, Electronics and Specialty Shapes, Ed. L. C. Klein (Noyes, Park Ridge, N.J., 1988) are of relevance and show the great importance attached to the formation of aerogels with specific properties and functions.
Examples of aerogels which contain or have added to them, metal ions or metal containing species are known. Those known fall into several categories, including: (1) a silica aerogel that has been dipped into a solution or dispersion containing the metal ion source; (2) a polymer matrix aerogel, such as a polyacrylonitrile aerogel, that contains metal ions added to the aerogel or to the gel before formation of the aerogel (e.g. L. M. Hair, L. Owens, T. Tillotson, M. Froba, J. Wong, G, J. Thomas, and D. L. Medlin, J. Non-Crystalline Solids, 186, 168 (1995), and S. Ye, A. K. Vijh, Z-Y Wang, and L. H. Dao, Can. J. Chem. 75, 1666 (1997)); or (3) a silica aerogel having metal ions (e.g. M. A. Cauqui, J. J. Calvino, G. Cifredo, L. Esquivias, and J. M. Rodriguez-Izquierdo, J. Non-Crystalline Solids, 147&148, 758 (1992)) or small metal compounds bound in it (e.g. Y. Yan, A. M. Buckley and M. Greenblatt, J. Non-Crystalline Solids, 180, 180 (1995)).
The use of supported metal and metal ions for catalysis is known, and many reports exist in the scientific, technical, engineering, and patent literature. Relevant studies include “The Chemistry of Ruthenium in PSSA Ionomer: Reactions of Ru—PSSA with CO, H2 and O2 and Alcohols” (I. W. Shim, V. D. Mattera, Jr., and W. M. Risen, Jr., Journal of Catalysis, 94, 531 (1985), which includes a report of a static Fischer—Tropsch reaction catalysis by supported ruthenium under mild conditions of 150° C. and 600 Torr total pressure, “A Kinetic Study of the Catalytic Oxidation of CO over Nafion-Supported Rhodium, Ruthenium, and Platinum”, by V. D. Mattera, Jr., D. M. Barnes, S. N. Chaudhuri, W. M. Risen, Jr., and R. D. Gonzalez, Journal of Physical Chemistry, 90, 4819 (1986), which shows this catalysis, and “Chemistry of Metals in Ionomers: Reactions of Rhodium-PSSA with CO, H2, and H2O”, Inorganic Chemistry, 23, 3597 (1984), which shows relationships between spectroscopic observations on supports that have been exposed to these gases and the compounds formed and the oxidation states of rhodium species.
Additionally, metal nanoparticles, formed as colloids by reactions in solutions and suspensions, are also known. They have been produced by chemical reductions and photolytically induced-reductions applied to solutions of salts of Au(III), Pt(II), Pd(II) and other metal ions. Chemical reductions are the most common, with such reactions as the citric acid reduction of Au(III) salts being among the most frequently used methods to obtain gold nanoparticles, but photolytic methods using γ-ray or ultraviolet irradiation have been reported as well. For example, ultra fine metal particles have been prepared by Itakura et al., T. Itakura, K. Torigoe and K. Esumi, Langmuir, 11, 4129-4134 (1995), by UV irradiation of salts dissolved in ethanol. The reaction is accelerated by using benzoin as a photoinitiator; indeed, the observed gold nanoparticle size changes from 7 nm to 17 nm with increasing benzoin concentration.
Nanoparticles have been formed in solid matrices of soft materials as well. For example, nanoparticles of Pt, Rh, Ru and Ag, have been obtained by chemical reduction in ionomer matrices, V. D. Mattera, Jr., D. M. Barnes, S. N. Chaudhuri, and W. M. Risen, Jr., J. Phys. Chem., 90, 4819 (1986); and, D. M. Barnes, S. N. Chaudhuri, G. D. Chryssikos, V. D. Mattera, Jr., S. L. Peluso, I. W. Shim, A. T. Tsatsas, and W. M. Risen, Jr., ACS Symposium Series No. 302, 66 (1986).
Studies of gold nanoparticles have been quite helpful for understanding molecular adsorption onto their surfaces and for utilizing these adsorption reactions for detection and other applications. In some cases, the adsorption reactions are used to control or at least found to control the sizes of the particles formed during the reduction process, because the adsorbed molecules can compete with the growth process for the addition of atoms to the surface of nucleated Au(0) clusters. For example, a gold sol was obtained by Pal, A. Pal, Talanta, 46, 583-587 (1998), by UV irradiation of HAuCl4 in aqueous Triton X-100, which acts as both a reductant and a stabilizer. Without it, gold (III) failed to form gold particles after extensive irradiation. In addition, Au colloids have been prepared by reduction of metal salts with UV irradiation in the presence of dendrimers. Thus, Esumi et al., K. Esumi, A. Suzuki, N. Aihara, K. Usui, and K. Torigoe, Langmuir 14, 3157-3159 (1998), studied their production in the presence of poly(amidoamine), which has surface amino groups. The average particle size decreased with increasing concentration of surface amino groups. At a surface amino group to HAuCl4 mole ratio of 1:1, the particle size was in the 2-18 nm range, with a broad distribution, but when the ratio was increased to 4:1, monodispersed gold particles of less than 1 nm were obtained because the colloids were protected by the dendrimers from aggregation.
Prior studies have shown that it is possible to obtain gold particles in aerogels by other methods, but none known to the inventors has lead to the light-induced production of Au(0) from coordinated Au(III) ions in an aerogel. Among the earlier studies is formation of noble metal clusters (Ag, Au) in a silica aerogel matrix by γ-irradiation of its hydrogel precursors loaded with an aqueous solution containing Ag ion or [AuCl4]− ions, J. F. Hund, M. F. Bertino, G. Zhang, C. Sotiriou-Leventis, N. Leventis, A. T. Tokuhiro, and J. Farmer, J. Phys. Chem. B, 107, 465-469 (2003). This work by Hund, et al. showed that clusters and particles in the 10 to 200 nm range could be formed. Gold nanoparticles dispersed inside the pores of monolithic mesoporous silica were prepared by soaking it in a gold (III) ion solution and subsequently subjecting it to ultrasonic irradiation. In that work by Fu et al., G. Fu, W. Cai, C. Kan, C. Li, and Q. Fang, J. Phys. D: Appl. Phys. 36 1382-1387 (2003), it was found that the nanoparticles actually were formed in solution first and then diffused into the pores. Anderson et al., M. A. Anderson, C. A. Morris, R. M. Stroud, C. I. Merzbecher, D. R. Rolison, Langmuir, 15, 674 (1999), presented a direct method to incorporate Au colloidal particles (either 5 or 28 nm) in a silica aerogel network structure by simply adding an Au sol, prepared by citrate reduction of HAuCl4, to a silica sol and then converting this to a composite.
Nanoparticles in colloidal suspensions or spread on surfaces, are available for further reaction, including reactions to carry out diagnostic assays and protein research. For instance, biotinylated thiol formed self-assembled monolayers on gold colloids and the composites produced were used for molecular recognition of avidin, based on the specific binding between biotin and avidin (streptavidin) (L. Haussling, H. Ringsdorf, F. J. Schmitt and W. Knoll, Langmuir, Vol. 7, No. 9 (1991) and C.-M. Pradier, M. Salmain, Z. Liu and C. Methivier, Surface and interface analysis 34, 67-71 (2002)). Niemeyer et al. (C. M. Niemeyer and B. Ceyhan, Angew. Chem. Int. Ed., 40, No. 19, 3685-3688 (2001)) reported a DNA-directed absorption of proteins on colloidal gold. This is part of an approach to using the protein coated gold compounds as reagents to detect proteins in an immunoassay. In the same vein, Mirkin and coworkers (R. Elghanian, J. J. Storhoff, R. C. Mucic, R. I. Letsinger, Chad A. Mirkin, Science, 277, 1078-1081 (1997)) reported a selective colorimetric polynucleotide detection method, based on the optical properties of gold nanoparticles.
There are a number of potential applications, especially in medicine, for small particles that are both ferromagnetic (or superparamagnetic) and can interact in such a selective manner with biologically active molecules. A particular one involves binding a molecule of interest in the blood or other physiologically interesting fluid system to the particles and then separating these particles from that system by magnetic separation. One approach to forming a material that will achieve this is to form magnetic particles, coat them in a biocompatible coating, such as Au or a polymer, and then derivatize the outer surface so that they will bind to the molecule(s) of interest (Berry, C. C and Custins, A. S. G., Phys. D: Appl. Phys 36(13) R198 (2003) and P, Gould, Materials Today, Feb 36 (2004)). Another application involves enhancement of magnetic resonance images (MRI) in diagnosis, while another involves selectively locating treatment agents, whether chemical or radiological, in particular areas to be treated.
As promising as the prior work has been, challenges remain. Some are associated with the size of nanoparticles and attendant issues, such as their removal and their potential deleterious transport in mammalian systems. Others have to do with the instability, indeed combustibility, of nanoscale metal particles in air, and their tendency to clump together.
In the present invention, a way to achieve the goal of obtaining a generally and selectively absorptive material that is ferromagnetic has been discovered. It combines (1) the generalized absorption capacity of silica-based polymer-silica hybrid aerogel with (2) the discovery that gold nanoparticles with specific adsorption capability can be produced directly by ultraviolet irradiation of Au ion-containing aerogels and (3) the discovery that well-dispersed ferromagnetic particles can be produced in these aerogels and their particles. The aerogel particles can be of any size from about the 100 nm range up to larger monolithic structures.
It also has been discovered that gold nanoparticles can be produced in or on the polymer-silica hybrid aerogel monoliths by ultraviolet irradiation so that spatially controlled arrays of such particles can be produced. It has further been discovered that gold ion-containing aerogel particles can be irradiated with ultraviolet light to produce aerogel particles comprising Au nanoparticles.
These and other aspects and/or objects of the disclosure are more particularly described below.